This invention relates generally to composite components and more particularly to the configuration of mounting features of composite components such as turbomachinery airfoils.
It is desirable to manufacture gas turbine components such as turbomachinery blades from composite materials that provide favorable strength-to-weight ratios. Known types of composite materials include polymer matrix composites (“PMC”), typically suitable for fan blades, and ceramic matrix composites (“CMC”), typically suitable for turbine blades.
All of these composite materials are comprised of a laminate of a matrix material and reinforcing fibers and are orthotropic to at least some degree, i.e. the material's tensile strength in the direction parallel to the length of the fibers (the “fiber direction”) is stronger than the tensile strength in the perpendicular direction (the “matrix” or “interlaminar” direction). The physical properties such as modulus and Poisson's ratio also differ between the fiber and matrix. The primary fiber direction in turbomachinery blades is typically aligned with the radial or spanwise direction in order to provide the greatest strength capability to carry the centripetal load imparted by the spinning rotor. As such, the weaker matrix, secondary or tertiary (i.e. non-primary) fiber direction is then orthogonal to the radial direction.
As composites have different coefficients of thermal expansion (“CTE”) than metal alloys use for the rotor disk, all of the blade dovetails use a configuration that allows for free thermal expansion between the two parts. However, this type of dovetail configuration leads to a peak interlaminar tensile stress imparted in the shank of the composite blade, which must be carried in the weaker matrix material, just above the pressure faces of the dovetail, commonly referred to as the “minimum neck”, which can be the limiting stress location in the blade design.
The matrix, or non-primary fiber direction strength, herein referred to as interlaminar strength, is typically weaker (i.e. 1/10 or less) than the fiber direction strength of a composite material system and can be the limiting design feature on composite blades, in particular, CMC turbine blades.
Accordingly, there is a need for a blade mounting structure which reduces interlaminar stresses in the mounting attachment region for a composite blade.